Referring to FIG. 1, the primary component of a typical metal interconnect layer 100 used in semiconductor circuits is aluminum. However, it is well known that aluminum migrates or diffuses into silicon, especially when an aluminum layer is in direct contact with a silicon substrate or region and the wafer 110 is subjected to high temperatures during processing of the wafer subsequent to the deposition of the metal layer. When using very small and thin semiconductor device structures, such as those which have become common in the late 1980's, the diffusion of aluminum into the silicon substrate causes undesirable leakage currents and device failures.
As shown in FIG. 1, a wafer 110 has a P- silicon substrate 120, in which has been formed a N+ region 122 (e.g., a source or drain region) which is bounded by field oxide 124 on either side. On top of the field oxide 124 is a thin intermediate oxide layer 126. A metal layer 100 interconnection to the N+region is then formed on top of the oxide layer 124-126.
One of the standard solutions to the problem of aluminum diffusion is to form a thin tungsten/titanium (W/Ti) layer 132 between an aluminum layer 134 and the substrate 120. The W/Ti layer 132 acts as a barrier which prevents diffusion of aluminum from the aluminum layer 134 into the substrate 120 (actually the N+region 122), thereby overcoming the aluminum diffusion problem. Similarly, a second tungsten/titanium (W/Ti) layer 136 is deposited on top of the aluminum layer 134. Frequently, grains of aluminum along the surface of an aluminum layer form "hillocks", i.e., vertically displaced grains, which are known to cause circuit failures. The second W/Ti layer 136 forms a compressive cap which helps to prevent hillocking of the aluminum. It also reduces the reflectivity of the entire metal sandwich 100, which facilitates photolithographic masking of the metal sandwich.
Referring to FIGS. 2 and 3, depositing W/Ti layers is, unfortunately, a very dirty process. FIG. 2 depicts a standard prior art sputter system 150, such as the Varian Model 3190 Sputter System. The sputter system 150 includes a load-lock port 152 for receiving wafers from an automatic wafer handling system (shown in part at reference numeral 154) and for returning wafers after processing back to the wafer handling system. The silicon wafers being processed are automatically removed from trays 156 of wafers which move along a track 158 as shown.
There are four stations 160, 162, 164 and 166 in the sputter system, each of which is used to perform a sequentially ordered processing step under the direction of a microprocessor controller 170. In other words, each wafer being processed automatically moves through these four stations 160-166 before being returned to the wafer tray 156 from which it came. Typically, one or more of the stations is used to perform preparatory steps, such as a plasma etch or wafer bake, and two or more of the stations are used for sputtering such materials as aluminum/copper (Al/Cu) or Tungsten/Titanium (W/Ti) onto a wafer. In the preferred embodiment, the aluminum/copper films deposited are approximately ninety-nine percent aluminum and one percent copper. The tungsten/titanium films deposited are approximately ninety percent tungsten and ten percent titanium.
FIG. 3 depicts one sputter station 162 of the sputter system 150 shown in FIG. 2. It should be understood that the sputter system 150 is designed so that any of the stations 160-166 of the system can be configured to perform any of the processing steps for which the system is designed, including heating, etching and sputtering. It should also be understood that all the stations 160-166 of the sputter system 150 are part of one evacuated chamber. The evacuated chamber is backfilled with argon at a partial pressure of about seven millitorr and is essentiallY devoid of air, with partial pressures for water and nitrogen each between 10.sup.-8 and 10.sup.-12 torr.
The one station 162 shown in FIG. 3 is configured for sputtering metal onto a target wafer. Station 162 includes a heater assembly 200 for heating and holding a wafer 202, as well as a metal source assembly 210. The metal source assembly 210 contains a metal cathode 212 which sputters metal onto the wafer 202. Unfortunately, metal from the source assembly 210 also sputters onto shielding 220 which surrounds the wafer 202 and which serves to protect the station 162 from becoming encrusted with sputtered metal. As will be understood by those skilled in the art, FIG. 3 only schematically represents the station's components and that the shielding 220 actually consists of several components.
FIG. 4 shows the configuration of a sputter system as used by the applicant prior to the present invention. This configuration is standard prior art. In particular, the first station 160 in the machine is used for an RF etch, which cleans the wafer prior to metal deposition. In the second station 162 twenty-two hundred (2200) angstroms of W/Ti are deposited on the wafer. Sputtering is performed in an argon atmosphere in which all or virtually all oxygen has been removed. Next, without exposing the wafer to air, an aluminum/copper film is deposited on the wafer by the third station 164 and then a second W/Ti layer (of about 825 angstroms) is deposited on top of the aluminum/copper film in the fourth station 166. These steps build up the circuit profile shown in FIG. 1.
After processing many wafers, there is a significant build up of sputtered metal inside the sputter chamber, particularly on the shielding 220 in the station 162. The deposition of Tungsten/Titanium films has always been a very dirty process. The high compressive stress of films deposited inside the sputter chamber causes flaking from the sputter chamber's internal components when the build up of W/Ti films reaches a critical level.
In particular, as the W/Ti films in a sputter system chamber approaches a thickness of 330 microns, the W/Ti film begins to relieve stress by flaking. This causes contamination of the wafers being processed, which causes product defects and requires that the sputter system be shut down for cleaning. During cleaning the shielding 220 in the sputter chamber is replaced, which requires disassembly of the sputter chamber--a time consuming process.
In general, using prior art manufacturing methods, cleaning of the W/Ti sputter chamber is an expensive process which must be repeated after processing about 1500 wafers, assuming a deposition of 2200 angstroms of W/Ti for each wafer.
It is a primary goal of the present invention to reduce the frequency with which sputter chambers used for depositing W/Ti films need to be cleaned. Related goal is to improve the quality of the W/Ti films deposited on wafers, and to improve the quality of the diffusion barrier provided by thin W/Ti films so that the thickness of the W/Ti films used can be decreased.